1. Field of the Invention
This invention relates to a pneumatic impact pulverizer for pulverizing powder materials by using air-jet streams (high-pressure gases), and a process for producing a toner which produces a toner for developing electrostatic images by means of the pulverizer.
2. Related Background Art
Toners or color resin powders for toners which are used in image forming methods carried out by electrophotography usually contain at least binder resins and colorants or magnetic powders. The toner develops an electrostatic image formed on a latent image bearing member, to form a toner image. The toner image thus formed is transferred to a recording medium such as plain paper or plastic film, and the toner image on the recording medium is fixed by a fixing means such as a heat fixing means, a pressure roller fixing means or a heat-pressure fixing means. Thus, the binder resins used in toners have the properties of undergoing plastic deformation upon application of heat or pressure.
At present, the toners or color resin powders for toners are prepared by melt-kneading a mixture containing a binder resin and a colorant or a magnetic powder (optionally further together with third components), cooling the resultant kneaded product, pulverizing the resultant cooled product and classifying the resultant pulverized product. The pulverization of the cooled product usually comprises crushing (or median pulverization) the cooled product by means of a mechanical impact pulverizer and subsequently finely pulverizing the crushed product by means of a pneumatic impact pulverizer making use of air-jet streams.
In the pneumatic impact pulverizer making use of air-jet streams, a powder material is carried on air-jet streams to form a particle-air mixture stream, which is then jetted out of an outlet of an accelerating tube, and this particle-air mixture stream is caused to collide against an impact member provided opposingly to the outlet of the accelerating tube so that the powder material is pulverized by the impact force.
As the above pneumatic impact pulverizer, machines as shown in FIGS. 16 and 17 have been used (Japanese Patent Application Laid-open No. 57-50554 and No. 58-143853).
In these pneumatic impact pulverizers, a powder material having a rough particle size is supplied from a hopper 22, and is sucked into an accelerating tube 1 through a powder material feed opening 24 communicating with the accelerating tube 1 at its middle portion, being sucked into the accelerating tube 1 by the action of a high-pressure gas fed through a high-pressure gas feed nozzle 25. The powder material thus sucked is jetted out of an outlet 10 of the accelerating tube 1 into a pulverization chamber 13 together with the high-pressure gas, is collided against an impact face 26 of an impact member 11 provided opposingly to the outlet 10, and is pulverized by the impact force. Then, the pulverized product is discharged out of the pulverization chamber 13 through a discharge outlet 14.
However, when the impact face 26 stands vertical to the axial direction of the accelerating tube 1 as shown in FIG. 16, the powder adjacent to the impact face 26 is in a high concentration and also the action of pulverization is chiefly the primary impact at the impact face 26, where the secondary impact against sidewalls 23 of the pulverization chamber is not effectively utilized, resulting in a low pulverization efficiency. In addition, when thermoplastic resin is pulverized, melt-deposits tend to occur on the impact face 26 because of local heat generation at the time of impact to cause a lowering of pulverization performance, making it difficult to achieve stable operation of the apparatus. Hence, it has been difficult to use the apparatus in the state of a high concentration for the powder to be fed into the accelerating tube.
In an instance where the impact face 26 is sloped by 45.degree. with the axial direction of the accelerating tube as in the case of the pneumatic impact pulverizer shown in FIG. 17, the above problems may occur less even when the thermoplastic resin is pulverized, and the powder in the vicinity of the impact face 26 can be in a lower concentration than in the case of the pulverizer shown in FIG. 16. However, the impact force used in pulverization when the powder is collided is smaller and also the secondary impact against sidewalls 23 of the pulverization chamber can not be effectively utilized, resulting in a pulverization performance which is lower by 1/2 to 1/1.5 than the pulverizer shown in FIG. 16.
A pneumatic impact pulverizer having solved the above problems is proposed as disclosed in Japanese Patent Application Laid-open No. 1-254266 and Japanese Utility Model Application Laid-open No. 1-148740.
The former Japanese Patent Application Laid-open No. 1-254266 discloses a proposal of a pneumatic impact pulverizer in which, as shown in FIG. 18, the impact face 26 of the impact member 11 has a specific conical shape so that the powder in the vicinity of the impact face can be in a lower concentration and yet can be collided against the sidewalls 23 of the pulverization chamber with good efficiency.
The latter Japanese Utility Model Application Laid-open No. 1-148740 discloses a proposal that, as shown in FIG. 19, a peripheral impact face 18 of the impact member 11 is so disposed as to be at right angles with the axis of the accelerating tube and a conical projection 17 is provided at its center so that the flow of powder can be prevented from being reflected on the impact face.
The pneumatic impact pulverizers shown in FIGS. 18 and 19 can overcome the above problems, but not to an extent that can be well satisfactory.
As a pneumatic impact pulverizer having better overcome the above problems, Japanese Patent Application Laid-open No. 5-309288 and No. 5-309287 disclose some proposals.
In the former Japanese Patent Application Laid-open No. 5-309288, as shown in FIG. 20, a pulverizing material fed through a pulverizing material feed tube 6 reaches a pulverizing material feed opening 5 formed between the inner wall of an accelerating tube throat 2 of the accelerating tube 1 and the outer wall of a high-pressure gas feed nozzle 3. Meanwhile, the high-pressure gas is jetted out of the high-pressure gas feed nozzle 3 toward an accelerating tube outlet 10. Here, the pulverizing material is sucked from the pulverizing material feed opening 5 toward the accelerating tube outlet 10 while being accompanied with the gas present together with the material, and is uniformly mixed with the high-pressure gas at the accelerating tube throat 2. Then the pulverizing material is collided against the impact face 26 of an impact member 11 provided opposingly to the accelerating tube outlet 10, which is collided in a uniform state free of uneven powder concentration, and second-order collided against a pulverization chamber sidewall 23 with good efficiency. Thus, the yield of the pulverized product and the pulverization efficiency per unit weight can be improved.
The latter Japanese Patent Application Laid-open No. 5-309287 discloses a proposal of an impact member 11 constituted of, as shown in FIG. 21, two impact areas formed of a projected central area 17 and a peripheral impact face 18. The first-order pulverized product of the pulverizing material pulverized at the projected central area 17 is second-order pulverized at the peripheral impact face 18. The pulverization chamber 13 has a pulverization chamber sidewall 23 for third-order pulverizing the secondary pulverized product second-order pulverized at the peripheral impact face 18.
The pneumatic impact pulverizers shown in FIGS. 20 and 21 can reasonably overcome the above problems. However, as recent needs, there is a demand for a more finely pulverized product and it is long-awaited to provide a pulverizer having a much better pulverization efficiency. Specifically, in image forming methods carried out by electrophotography, it is desired to make toner particle diameter smaller in order to achieve a higher image quality and it is long-awaited to provide a process for producing toners with much better efficiency.